During an ophthalmic laser surgical procedure, wherein stromal tissue within the cornea is ablated, the ablation is caused by an effect known as Laser Induced Optical Breakdown (LIOB). Typically, LIOB in the stroma is accomplished using pulsed laser beams that may have pulse repetition rates as high as 10 KHz. In detail, the LIOB effect of successive individual laser pulses is cumulative. Each individual laser pulse, however, can be considered separately.
For an individual laser pulse, it happens during LIOB that the tissue being ablated may be subjected to several different phenomena. For one, tissue that is peripheral to the ablated tissue is subject to adverse side effects, such as tearing (mechanical damage) and scorching (thermal damage). It is known, however, that these particular adverse side effects can be avoided if the pulse energy density is minimized. On the other hand, the pulse energy density must be above the tissue threshold in order for LIOB to occur. With these countervailing considerations in mind, it has been determined that a laser pulse having the following characteristics can cause LIOB in stromal tissue, while avoiding adverse mechanical or thermal side effects on peripheral tissue.
Laser Pulse                Pulse Length (duration): 1-1000 femtoseconds        Energy Density: 1-10 J/cm2        Focal Spot Size: 1-10 μm diameter        Pulse Repetition Rate: multi KHz        
Despite the adverse, but avoidable, side effects on peripheral tissue noted above, LIOB will still affect stromal tissue in at least three other different, identifiable ways. These are: 1) plasma formation; 2) shock wave generation; and 3) cavitation bubbles. Schematically, these three phenomena are shown in FIG. 1 of the drawings.
Referring for the moment to FIG. 1 in the drawings, the consequences of LIOB caused by a single laser pulse are illustrated in a spatial context. It is to be appreciated, however, these consequences also have a temporal context. First, a micro plasma is formed from tissue located within the focal spot of the laser pulse. Specifically, this plasma results from the evaporation of corneal tissue 10 in a tissue volume 12 that has a diameter “d1” in the range of around 1-10 microns (d1=1-10 μm). The formation of this plasma is then followed by a shock wave that radiates through the tissue 10. Typically, the shock wave extends from the center of volume 12 through a radius “r” that is approximately twenty microns (r≅20 μm). The shock wave, however, decays within a few nanoseconds. Nevertheless, despite its relatively short duration, the shockwave effect should be kept as small as possible by using pulse energies that are not too far above the threshold for LIOB.
Perhaps, the most pronounced adverse effect from LIOB at relatively low pulse energies is the creation of a cavitation bubble 14. Stated differently, at relatively low pulse energies there is typically no mechanical or thermal damage to peripheral tissue. Instead, a laser pulse having the parameters set forth above will induce LIOB that immediately results in a cavitation bubble 14 (see FIG. 1). There it will be seen that the bubble 14 has a diameter “d2” that will generally be greater than about twice the diameter “d1” of the tissue volume 12 (d2≧2d1). Although the cavitation bubble 14 will eventually decay, as generally indicated in FIG. 2, it has a time dependence that should be accounted for (N.B. FIG. 2 is only exemplary). In particular, FIG. 2 indicates the temporal influence of a cavitation bubble 14 may be considered as continuing through two decay periods. Specifically, the decay of the bubble 14 experiences a first relaxation rate of approximately 10 microns per second (10 μm/sec) during a first decay period, of time “τ”. During “τ” the bubble 14 decays to a diameter “d3” which is less than “d2” but greater than “d1” (d1<d2>d3, with d3>d1). Typically, the period “τ” is in the range of about 1-1000 μs and depends on a number of factors including pulse energy density. Thereafter, during a second decay period, the bubble 14 fully dissipates from the diameter “d3” in about 15 to 30 minutes at a second relaxation rate of approximately half a micron per minute (0.5 μm/min).
In light of the above, it is an object of the present invention to provide a system and method for performing laser induced optical breakdown (LIOB) in a substantially transparent material (i.e. the cornea of an eye) wherein a predetermined time period “τ” is interposed between adjacent laser focal spots in a spot pattern. Another object of the present invention is to provide a system and method for performing laser induced optical breakdown (LIOB) in a substantially transparent material (i.e. the cornea of an eye) wherein a pattern of successive focal spots are both spatially and temporally separated from each other. Yet another object of the present invention is to provide a system and method for performing laser induced optical breakdown (LIOB) in a substantially transparent material (i.e. the cornea of an eye) wherein LIOB is induced at a location where the residual influence of earlier LIOB is effectively avoided. Still another object of the present invention is to provide a system and method for performing laser induced optical breakdown (LIOB) in a substantially transparent material (i.e. the cornea of an eye) which is easy to use, relatively simple to manufacture, and comparatively cost effective.